It is proposed to continue the analysis of the P2-P4 system consisting of temperate bacteriophages P2 (or a P2 relative) serves as helper for its satellite P4. The main aim is to further elucidate the regulatory interactions controlling gene expression in this system and especially the regulatory coupling between helper and satellite. Specific objectives include: (1) further analysis of cox mutations of P2 with regard to their effects on depression of P2 and P4 and in promoted recombination; (2) analysis of P4 mutations enabling P4 to grow in cells lysogenic for a P2 related phage not accessible as helper to wild type P4; (3) study of P4 mutations bacterial growth even in the absence of a helper; (4) analysis of P2 mutations affecting phage head size determination; (5) search for P2-P4 DNA hybrids (if they exist they could be of crucial importance for P2-P4 interactions); (6) search for new P2 and P4 mutants that affect control of gene expression in the P2-P4 system.